1. Field of the Invention
The present invention relates to an ion source vaporizer for heating a solid sample of an ion source supplied to an ion beam irradiation apparatus such as an ion implantation apparatus. More particularly, the present invention relates to anion source vaporizer which can prevent a clogging of a nozzle for feeding a vapor sample from the vaporizer to an arc chamber.
2. Description of the Related Art
In these days, the surface modification with ion irradiation and the implantation of impurities into a silicon wafer or a glass substrate are actively conducted. These ions are obtained by ionizing a gas or a solid sample. The metal such as phosphorus, antimony or aluminum is solid at the ordinary temperature. To obtain ions from those solids, it is required that the solid sample is vaporized by heating and fed in vapor state into an arc chamber.
Usually, to produce a vapor from the solid sample, an ion source vaporizer is employed having a hollow sample cell and a heater for heating and evaporating the solid sample around it.
FIG. 2 shows one example of the conventional ion source vaporizer. An ion source vaporizer 1 includes an vaporizer main body 2, a nozzle 10, a nozzle fixing member 20 and a heater 30.
The vaporizer main body 2 is made of a metal such as stainless steel, and has a hollow structure with a bottom having an opening portion. The vaporizer main body 2 has a sample cell 3 on the bottom side of the hollow structure, a taper portion 4 in the central portion and a thread groove 5 in the upper portion. The sample cell 3 and an arc chamber 40 are connected through the nozzle 10 made of metal such as stainless steel. The nozzle 10 has a collar portion 11 is formed at one end portion thereof. The collar portion 11 is fixed to the taper portion 4 in a manner to cover the sample cell 3 by the nozzle fixing member 20. The nozzle fixing member 20 has a thread ridge 21 around a part of the outer periphery thereof and the thread ridge 21 is mated with the thread groove 5. The other end of the nozzle 10 is fitted into a gas inlet port, not shown, of the arc chamber 40. A solid sample 31 is ion species such as phosphorus or antimony. A temperature measuring device 32 such as a thermocouple is provided near the bottom of the sample cell 3. The heater 30 such as a sheath heater is provided around the vaporizer main body 2, and connected via a cable, not shown, to a power source. Furthermore, a depression portion 6 is provided in the bottom portion of the vaporizer main body 2. When the solid sample 31 is exchanged, it is required that the ion source vaporizer 1 is quickly cooled. To do this, the air is forced into the depression portion 6 from the compressed air supply, not shown.
The vaporizer main body 2 and the sample cell 3 are heated by energizing the heater 30, so that the solid sample 31 filled within the sample cell 3 is vaporized. A vapor produced in the sample cell 3 passes through a nozzle guide portion 12 of the nozzle 10 and fed into the arc chamber 40. A signal of the temperature measuring device 32 is sent to a controller, not shown, which controls a current applied to the heater on the basis of the signal to regulate the temperature of the sample cell 3.
In the ion source vaporizer 1 as described above, the taper portion 4 is formed as a result of machining requirement of the vaporizer main body 2. The collar portion 11 of the nozzle 10 is fixed to the taper portion 4 of the vaporizer main body 2, and the outer circumference of the collar portion 11 and the taper portion 4 are only in linear contact (more strictly in point contact at several points). Therefore, there is a great thermal resistance between the collar portion 11 and the taper portion 4, so that the heat of the vaporizer main body 2 is not sufficiently transmitted to the collar portion 11.
The arc chamber 40 has normally a higher temperature (e.g., 600xc2x0 C.) than the sample cell 3, so that the heat is transmitted from the arc chamber 40 to the nozzle 10. However, the heat from the arc chamber 40 is mostly transmitted via the nozzle fixing member 20 in contact with the nozzle 10 to the vaporizer main body 2, not to the nozzle 10. Therefore, heat is difficult to be transmitted especially near the collar portion 11 of the nozzle 10 from the vaporizer main body 2, and further from the arc chamber 40. Hence, it is estimated in a certain ion source operation condition that the collar portion 11 has the lowest temperature in vapor transmission paths of the ion source vaporizer 1.
Consequently, a vapor of the solid sample 31 produced in the sample cell 3 is cooled on the way to the arc chamber 40 by the collar portion 11 having lower temperature, recrystallized and grown (a vapor coming later is further recrystallized), leading nozzle to a clogging problem.
Every time the nozzle 10 is clogged, the operation of an ion source must be stopped to replace the ion source vaporizer 1, resulting in a lower throughput of the ion implantation apparatus. Also, the ion source vaporizer 1 having the nozzle 10 clogged must be overhauled and cleaned, resulting in a problem in its overhaul.
When the ion species is antimony especially, this problem is worse. A melting point of antimony is about 630xc2x0 C., and if the temperature of the sample cell 3 is increased above the melting point, antimony is completely melted within the sample cell 3 and becomes liquid. If the ion source is used in such a condition that the ion species is used in such a liquid state, liquid antimony sticks to the sample cell 3, the nozzle 10, and the arc chamber 40, causing a cleaning problem. Hence, when antimony is employed as the ion species, the temperature of the sample cell 3 must be set at the melting point or below.
FIG. 3 shows a conceptual graph representing the temperature distributions of the arc chamber 40, the nozzle 10 and the vaporizer main body 2 (sample cell 3). The line X indicates the temperature distribution of the nozzle 10. The temperature of the nozzle 10 is highest (A) at a portion fitted into the arc chamber 40, is decreased as being farther away from the arc chamber 40, and lowest (B) in the collar portion 11. This is because the collar portion 11 of the nozzle 10 is less subject to thermal conduction from other parts, as previously described. Herein, when the temperature of the sample cell 3 is set at a temperature (C) below the temperature (B), namely, on the line Y, if a sufficient amount of vapor can be collected, a vapor produced in the sample cell 3 does not come into contact with a lower temperature portion than the temperature (C) of the sample cell 3 till it reaches the arc chamber 40, and is not recrystallized. However, in a case where the solid sample 31 is antimony, a sufficient amount of vapor can not be collected if the temperature of the sample cell 3 is lower than the temperature (B). Hence, the temperature of the sample cell 3 must be set at a higher temperature (D) than (B). In this case, the vapor at the temperature (D), namely, on the line Z, comes into contact with the collar portion 11 of the nozzle 10 at lower temperature (B), so that the vapor temperature is decreased, and antimony is recrystallized in the collar portion 11. Therefore, the nozzle is clogged. A similar phenomenon is seen when the solid sample 31 is aluminum (melting point 660xc2x0 C.).
Thus, it is an object of the present invention to provide an ion source vaporizer that can draw an ion beam stably for a long time, when antimony or aluminum is employed as the solid sample, in which antimony or aluminum vaporized once in the sample cell is not recrystallized in a nozzle to cause the clogging of the nozzle.
According to the present invention, there is provided an ion source vaporizer comprising: a hollow vaporizer main body having an opening portion; a heater for evaporating a solid sample within the vaporizer main body; a nozzle for feeding a vapor produced within the vaporizer main body into an arc chamber; a crucible for filling the solid sample, the crucible being disposed within a cavity of the vaporizer main body and being engaged with the nozzle; and a pressing unit for pressing the crucible against the vaporizer main body. In the ion source vaporizer, preferably, an upper part of the crucible is screwed one end portion of the nozzle, and a bottom part of the crucible is pressed against the bottom of the cavity of the vaporizer main body by the pressing unit.
With the above constitution, the crucible and the nozzle are engaged together, whereby the thermal resistance from the crucible to the nozzle can be reduced. Since the temperature of a part of the nozzle, especially, the collar portion of the nozzle where the temperature is likely to be lower can be almost equal to the vapor temperature, the recrystallization of vapor on the collar portion of the nozzle, namely, clogging of the nozzle can be prevented.